The present invention pertains to several techniques for improving coating adhesion to wooden articles, such as railroad crossties, pilings, boat docks, decks, porch and patio flooring, fences, telephone poles, and many other wooden articles of various cross sections.
The present invention is particularly directed to improving coating adhesion for railroad crossties that are coated by a continuous process involving the passage of a series of wooden members, having end caps, in a substantially end-to-end configuration through a crosshead extrusion die. Such process extrudes resinous molten extrudate from a distribution passage into the die opening and around the side surfaces of the wooden member so as to form a coating on the side surfaces.
Various types of railroad crossties are known in the art. Examples of such railroads crossties are shown in U.S. Pat. Nos. 952,977; 1,036,860; 1,041,736; 1,623,158; and 5,916,932.
The useful life of railroad crossties has been extended by coating a wooden core member with a resinous coating and by placing end caps over the ends of the core member. Such technique is described in U.S. Pat. No. 6,336,265, granted to Niedermair on Jan. 8, 2002. This patent describes a composite railroad crosstie for supporting railroad track rails on a ballast or concrete roadbed. The crosstie comprises a wooden core of virgin or recycle natural wood or of man-made, engineered wood such as oriented strand board (OSB), plywood, and the like. The wooden core is coated with virgin or recycled thermoplastic resins, thermosetting resins, and/or rubber. Fillers or reinforcements may optionally be included. During manufacture, the core member is sized to a dimension less than the desired dimension of the finished crosstie to provide space for the coating. End caps are positioned over the ends of the core member and then a resinous coating is applied to the core member in a continuous process by passing a series of core members, containing end caps, in a substantially end-to-end configuration through a cross head die.
Although the above-described composite crosstie constitutes an improvement over uncoated wooden crossties, problems in coating separation from the core member have been encountered. Such separation serves to reduce the useful life of the railroad crosstie because the beneficial effect of the protective coating is minimized, or even lost. The present invention addresses this problem in the art by treating the surface of the core member prior to coating to substantially improve coating adhesion and thereby achieve a crosstie having an extended useful life. Surface treatments include placing grooves on the surface of the core member and/or driving off moisture from the surface of the core member and/or incising prior to coating the surface of the core member. When used in combination, grooving and incision are normally performed prior to driving off moisture from the surface of the core member, followed by subsequent coating. These techniques serve to significantly improve adhesion between the wooden crosstie and its coating.